First cut of Inactivity locking when a Security PIN is set

This will lock the screen whenever the current user has a security PIN set.  Full preferences support for the inactivity amount.

There is a m.ViewController.IsLocked = false/true that is set whenever the system is locked. Currently unused though.

This seems fully functional but should have additional testing but as I won't be able to get back to it for a few days  I'm sharing this now.